This invention relates to an ink jet print head and more particularly to an ink jet print head for generating ink drop on demand under control of a suitable electrical signal.
Ink jet printing has been known in the prior art, including systems which use a pressure generated continuous stream of ink, which is broken into individual drops by a continuously energized transducer. The individual drops are selectively charged and deflected either to the print medium for printing or to a sump where the drops are collected and recirculated. Examples of these pressurized systems include U.S. Pat. No. 3,596,275 to Sweet, and U.S. Pat. No. 3,373,437 to Sweet et al. There have also been known in the prior art ink jet printing systems in which a transducer is used to generate ink drops on demand. One example of such a system is commonly assigned U.S. Pat. No. 3,787,884 to Demer. In this system the ink is supplied to a cavity by gravity flow and a transducer mounted in the back of the cavity produces motion when energized by an appropriate voltage pulse, which results in the generation of an ink drop. A different embodiment of a drop-on-demand system in which the transducer is radially arranged is shown in U.S. Pat. No. 3,683,212 to Zoltan.
The prior art drop-on-demand printing systems have been limited by low drop production rate, by a low efficiency and by a jet instability which produced drops with irregular spacing and/or size which lead to poor print quality as the drop rate was increased. One reason for the low drop production rate in prior art drop-on-demand printing systems is the time required to replenish the ink after ejection of a drop, and a second reason is that, to prevent unwanted ink drop satellite formation, complete damping of the internal fluid oscillations within the ink must be attained before drop ejection can be repeated. A basic reason for the low efficiency of prior art drop-on-demand printing systems is that, during the operational cycle of a drop-on-demand print head, ink is moved not only in the downstream direction toward the nozzle, but also in the upstream direction toward the ink supply. If the impedance in the upstream supply line is much smaller than that in the nozzle, most of the kinetic energy generated in the head is used to accelerate the ink toward the ink supply and only a small fraction of the generated kinetic energy is used to eject droplets out of the nozzle. If the impedance of the upstream supply line is made much higher than that of the nozzle, then ink cannot be resupplied fast enough to the ink cavity, and the drop-on-demand print head will not operate properly. To avoid either of the limiting cases, the impedance of the upstream and downstream fluid line has been generally chosen to be of the same order of magnitude. This implies that the efficiency of the prior art drop-on-demand print heads is substantially below optimum efficiency.